Evolutionary psychologists get a bad rap. I should not be surprised, really. They probe motivations for human behaviour that often exist far beneath conscious thought and the sanitised stories people tell themselves about why they do what they do. Some evolutionary explanations for human behaviour sound so outrageous that the only reasonable reaction seems to be … outrage.

In this vein, I recently enjoyed a hugely productive detour into the science of social conservatism. Not to say that conservatism is a science. That would be ironic. I mean the evolutionary explanations for why some people gravitate toward conservatism and others toward progressive ideas.

Haidt studies moral intuitions which bias our responses to particular situations in predictable ways. The six main intuitions, or moral foundations as Haidt and his colleagues have called them are: a need to protect others from harm; a sense of fairness; loyalty and willingness to make sacrifices for one’s group; an impulse toward liberty from oppression; a respect for authority; and purity, the idea that certain sacred or pure things should be preserved.

In Haidt’s studies, conservatives respond strongly to each of these six intuitions, whereas progressives have a different idea of liberty, less respect for authority and a weaker intuition for purity. They also concern themselves less about their own group and extend their compassion to a wider group.

Evolutionary biologists have known for some time that few agents exact as strong selection on a population as pathogens and parasites do. Immune genes are almost always among the fastest-evolving genes in a population because hosts and their parasites lock one another into a perennial arms race of offense and defence. Jared Diamond’s Guns, Germs & Steel remains a fascinating introduction to the large-scale consequences of disease for recent human evolution and the history of society.

Schaller’s inspired suggestion was that certain behaviours allow us to detect cues of likely infection, respond to those cues and thus avoid pathogen infection. These adaptations, together, comprise the Behavioural Immune System (BIS). Not really an immune system, the BIS is more a suite of psychological adaptations that tend to reduce the chances of infection.

Disgust, very much the fashionable emotion du jour, serves as an important component of the BIS. So does a general distrust of strangers. That’s because in our long history of living in small groups where everyone knew each other, new infections usually came from outside.

If the BIS so effectively helps people avoid new diseases, then why don’t we all just turn it up to 11? Well, if one can transcend disgust, contact with strangers has its benefits. Sex, for example, rather depends on suspending disgust in order to share bodily fluids. Equally importantly, trade would become impossible if we were not able to overcome our fear of strangers and any pathogens they might bear.

And so the BIS varies in strength, from person to person and situation to situation. When cues of contagion are rife, people become more insular, less open to new experience and far less trusting of strangers. Pictures of vomit, blood or faeces, or the smell of garbage, not only affect disgust, they also prompt people to behave more conservatively.

Enter stage right

The tendencies to adhere to tradition, submit to authority and conform to hierarchy are all part of the socially conservative repertoire. As is a strong tendency to act unwelcomingly or aggressively toward out-group members. All of these behaviours promote in-group cohesion and negativity toward out-group members. Exactly the kinds of behaviours, then, that would minimise the risks of infection during an epidemic.

In the few short years since Schaller first mooted the idea of the Behavioural Immune System, studies claiming all manner of links between the BIS and conservative impulses have cropped up. Disgust, it seems, is an important component of the mistrust that right-wing authoritarians show toward foreigners and the profound ease that religious conservatives feel toward homosexuals and transexuals.

Individuals with stronger BIS responses tend also to be less open (openness being one of the Big Five personality dimensions) to new experiences and to variation in sociosexuality. They also tend to be more strongly socially and politically conservative. And countries where disease is more prevalent and healthcare less adequate tend also to be more socially, sexually and politically conservative, even after controlling for economic differences.

And yet despite the steady stream of evidence, to which I have only paid passing attention, the links between socially and morally conservative impulses and behaviours that protect individuals from infection seemed to me to be among the more fantastic claims emanating from evolutionary psychology. Was this stuff the real deal, or perhaps it was the kind of flimsy stuff that defies replication and is soon forgotten?

To that end, I was pleased today to discover in the journal Evolution & Human Behavior a meta-analysis of the studies performed thus far testing the links between social conservatism and the behavioural immune system. Meta-analyses provide invaluable opportunities to assess the overall forest of results without being distracted by any particularly spectacular trees - the news-making single studies.

This meta-analysis, led by John A Terrizi, assembled the 24 studies that have been performed in this area in the last five years. The results suggest that the links between the BIS and socially conservative attitudes and behaviour are real and solid.

According to the authors:

The results indicate that behavioral immune strength, as indicated by fear of contamination and disgust sensitivity, is positively related to social conservatism (i.e., right-wing authoritarianism, social dominance orientation, religious fundamentalism, ethnocentrism, collectivism, and political conservatism). These findings provide initial evidence that socially conservative values may function as evolutionarily evoked disease-avoidance strategies.

So, it seems conservatism has its uses. Or had its uses in our evolutionary past. Perhaps that’s why US Republicans so abhor President Obama’sAffordable Care Act? It may, literally, present the cure for Republican social conservatism.